(1) Field of the Invention
The present invention relates to an electronically controlled ignition device provided for a combustion engine so as to control the ignition timing and the actuating time duration for the ignition coil thereof with response to the running condition thereof.
(2) Description of the Prior Art
The optimum ignition timing for the combustion engine is shifted with response to the revolution of the engine, the load imposed thereupon, the state of the cooling water and the inhaled air and so on, that is, the running conditions thereof. Among these conditions, the revolution thereof affects the ignition timing particularly, so that the more the revolution thereof is increased, the sooner the optimum ignition timing is advanced.
On the other hand, the high tension required for the ignition is generated on a secondary side of the ignition coil at an instant at which a primary current supplied thereto is cut off. In this case, a certain time duration elapsing after the ignition coil is energized is necessary for the amount of the primary current to reach a sufficient value enough to generate the high tension required for the ignition on the secondary side thereof because of the inductance thereof. Accordingly, it is necessary for effecting the ignition at the optimum ignition timing for the combustion engine to secure the aforesaid certain time duration required for the primary current to be grown up to the sufficient amount after the ignition coil has been energized prior to the optimum ignition timing. That is, although it is necessary as mentioned above that the ignition timing is shifted with response to the running conditions of the engine, the time duration for actuating the ignition coil should be maintained at a constant value.
Based on the mentioned above, in the conventional electronically controlled ignition device, an unactuated time duration required for maintaining the actuating time duration for the ignition coil at the constant value is calculated as well as the ignition timing is settled with response to the running condition of the combustion engine, so as to start the actuation of the ignition coil when the aforesaid unactuated time duration has expired after the application thereupon of the ignition signal.
However, such defects are apt to be caused according to the mentioned above that the misfire is brought about on a transient condition based on the too short actuating time duration, whilst an overload based on the too long actuating time duration is imposed on a driving transistor for the primary current to be supplied to the ignition coil.
The above mentioned defects will be explained hereinafter by referring to FIG. 1.
In FIG. 1, the ignition signal is indicated by (a), the unactuated time duration is indicated by (b), the actuating time duration is indicated by (c) and further the primary current supplied to the ignition coil is indicated by (d). For example, in the case that the speed of revolution of the engine is shifted from a low speed to a high speed, the higher the speed of revolution is raised, the sooner the ignition timing is advanced, so that the interval of the ignition signals is shortened on account of the advance of the ignition timing as shown in a section (A) of FIG. 1. However, regardless of the above shortened interval of the ignition signals, since the unactuated time duration is determined on the base of low speed of revolution of the engine prior to the above shift of speed, the actuation for the ignition coil is started at an instant at which the unactuated time duration t.sub.1 on the low speed condition has expired after the preceding ignition timing, so that the actuating time duration is shortened. As a result thereof, as shown by (d) in FIG. 1, the actuation is cut off before the primary current reaches to the sufficient amount thereof.
On the contrary, in the case that the speed of revolution of the engine is shifted from a high speed to a low speed, the interval of the ignition signals is elongated as shown in a section (B) of FIG. 1. However, regardless of the above elongated interval of the ignition signals, the actuation for the ignition coil is started at an instant at which the unactuated time duration t.sub.2 on the high speed condition has expired after the preceding ignition timing, so that the actuating time duration is elongated. As a result thereof, as shown by (d) in FIG. 1, the primary current supplied to the ignition coil reaches to an excessive amount.
According to the mentioned above, in the conventional electronically controlled ignition device, such a controlling method has been employed for the purpose of enabling the shift of the timing, at which the actuation of the ignition coil is started, with response to the shift of the actual ignition timing that the timings at which the actuation of the ignition coil is started and cut off are controlled with the time duration or the angle of the revolution of the engine, which is measured from a predetermined reference position of revolution.
However, according to the above mentioned method, when the speed of revolution of the engine reaches to the high speed range, it is feared that the timing at which the actuation of the ignition coil should precede the aforesaid reference position, in order to secure the constant actuating time duration for the ignition coil, so that it is required that different reference positions of revolution are settled respectively for the high and low speed ranges. As a result thereof, the defect of complicated configuration is caused.